The art of transmitting an item of binary information reduces itself to the transmission of one or more binary bits where each such binary bit represents either a single binary digit of "1" or a single binary digit of "0" and where the number and arrangement of such binary bits per item is determined by the binary number to be represented by each item.
The following U.S. Pat. Nos. are representative of the prior art where each binary bit is represented by a single direct current signal during a transmission time slot TS per bit and where a plurality of successive such time slots TS are required to represent a binary number consisting of more than one binary digit: 2,759,047 to L. A. Meacham of Aug. 14, 1956; 2,996,578 to F. T. Andrews, Jr. of Aug. 15, 1961; 3,057,962 to H. Mann et al. of Oct. 9, 1962; 3,133,280 to T. V. Crater of May 12, 1964; 3,302,193 to J. M. Sipress of Jan. 31, 1967; and 3,502,810 to M. R. Aaron et al. of Mar. 24, 1970. In such prior art, the single signal during each time slot TS is either a zero level signal, a single finite level signal of positive polarity (+) with respect to the zero level, or a single finite level signal of negative polarity (-) with respect to the zero level. In Meacham, + is used to represent binary 1 and - is used to represent binary 0; and, zero is used to represent consecutive 1s or 0s during a plurality of successive time slots TS. Andrews uses zero to represent binary 0 and either + or - to represent binary 1, the + and - being used alternately to represent consecutive 1s during a plurality of successive time slots TS. In Mann et al., the same scheme is used as in Andrews except that the end of a plurality of successive time slots TS is signified, in a time slot TS following the last time slot TS of the plurality, by the use of the same + or - as the last previous binary 1. Crater uses zero to represent binary 0 and either + or - to represent binary 1, the + being used if an even number (that is, 0, 2, 4, et cetera) of time slots TS occur since the last binary 1, and the - being used if an odd number (that is, 1, 3, 5, et cetera) of time slots TS occur since the last binary 1. In Sipress, binary 1 is represented by either + or - and binary 0 is represented by either +, - or zero, the choices being made over a plurality of successive time slots TS according to a coding scheme. Aaron et al. uses either +, - or zero to represent either binary 1 or binary 0, zero being used to represent binary 0 where less than four consecutive binary 0s occur during a plurality of successive time slots TS and where otherwise the choices are made over the plurality of successive time slots TS according to a coding scheme.
U.S. Pat. No. 4,002,833 to J. J. Eachus of Jan. 11, 1977 is another type of prior art wherein each binary bit is represented by a single direct current signal during a transmission time slot TS per bit and particularly wherein two bilevel signals are used such that any change of only one signal between levels represents a binary 1 or a binary 0 and a change between levels of both signals at the same time represents the end of a group of successive bits.
The following U.S. Pat. Nos. are representative of the prior art where each binary bit is represented by two successive direct current signals during two successive time segments T1 and T2 constituting a transmission time slot TS per bit (that is, TS=T1+T2) and where a plurality of successive such time slots TS are required to represent a binary number consisting of more than one binary digit: 2,700,696 to R. H. Barker of Jan. 25, 1955; 3,216,008 to F. R. Goldammer of Nov. 2, 1965; 3,403,377 to T. A. Connolly et al. of Sept. 24, 1968; 3,678,194 to I. F. Orrell, Jr. of July 18, 1972; and 3,808,366 to C. M. Wanamaker of Apr. 30, 1974. In Barker, Goldammer and Wanamaker each of the two signals during time segments T1 and T2 of time slot TS is either a zero level signal, a single positive (+) level signal, or a single negative (-) level signal. In Barker the signal during time segment T1 is zero to represent binary 0, the signal during time segment T2 is zero to signify the end of the time slot TS, and during a plurality of successive time slots TS the signal during time segment T1 is alternately + and - to represent successive binary 1s. Goldammer provides a + during time segment T1 as a synchronizing signal and provides respective - and zero during time segment T2 to represent respective binary 1 and binary 0. In Wanamaker, time segment T1 uses a - signal to represent binary 1 and a zero signal to represent binary 0. Connolly et al. uses only the two signal levels of + and zero, the time segment T1 signal being + to represent binary 1 and zero to represent binary 0 with the time segment T2 signal being the opposite of the time segment T1 signal. In Orrell the four signal levels of zero, +1, +2 and +3 are used, the time segment T1 signal being +2 as a synchronizing signal, the time segment T2 signal being +1 to represent binary 1 and zero to represent binary 0, and the time segment T1 following the time slot TS for the last of a plurality of successive time slots TS contains a +3 signal to signify the end of the plurality of time slots TS.
U.S. Pat. No. 3,369,229 to I. Dorros of Feb. 13, 1968 represents the prior art where a binary number consisting of more than one binary digit is represented by two successive direct current signals during two successive time segments T1 and T2 constituting a transmission time slot TS. Dorros uses the six signal levels of +5, +3, +1, -1, -3 and -5 one at a time during each of time segments T1 and T2 according to a coding scheme to represent various four binary digit numbers.
U.S. Pat. No. 3,292,147 to J. Dascotte of Dec. 13, 1966 represents the type of prior art where each binary bit of 1 and 0 is represented by three successive signals during three successive time segments T1, T2 and T3 constituting a transmission time slot TS per bit. In Dascotte, three different alternating current frequencies A, B and C are used with a different one of the three frequencies occurring during a different one of the time segments T1, T2 and T2 and with two different sequences of such occurrences representing the two values of binary 1 or binary 0. Specifically, for the sequence of time segments T1 to T2 to T3 any one of the sequences A to B to C, B to C to A and C to A to B represents binary 0 and any one of the sequences A to C to B, C to B to A and B to A to C represents binary 1.
U.S. Pat. No. 4,020,282 to P. H. Halpern of Apr. 26, 1977 represents the type of prior art where a binary number consisting of more than one binary digit is represented by more than two successive direct current signals during the same number of successive time segments constituting a transmission time slot TS. Halpern uses a time slot TS consisting of six time segments T1 through T6 and uses the two signals of + and - occurring during time segments T1 through T6 according to a coding scheme part of which requires that the six signals consist of three + and three - signals.
The large number of ways devised in the prior art for transmitting one or more binary bits reflects a continuing search for methods of transmission which can solve the many problems confronting the use of direct current signal formats. The following seven such problems are significant problems among perhaps others not mentioned: (1) the desirability of providing a signal format which reduces or eliminates the tendency for center-line-drift to occur; (2) the desirability of including synchronizing information in the signal format; (3) the desirability of providing a signal format which requires neither a time clock nor a time slot recovery means: (4) the desirability of providing a signal format which is repeatable by self-timed repeaters; (5) the desirability of providing a signal format which tolerates large variations in transmission bit rate; (6) the desirability of providing a signal format which includes end-of-item information and which includes end-of-group information where a group of items are transmitted; and, (7) the desirability of providing a signal format which permits a transmitted item of information to represent plural digit binary numbers and which permits a group of such items to be transmitted.
The above-cited examples of the prior art teach methods which solve one or more but not all of the foregoing problems.